When high pressure gases, such as hydrogen (“H2”) and compressed natural gas (CNG) are used as fuels in motor vehicles, a fuel depot infrastructure for efficient consumer refueling of the vehicles must also be developed. Typically, in the use of hydrogen gas to power fuel cells, or in the use of compressed natural gas, to power internal combustion engines in motor vehicles, present practice is that high pressure fuel is stored is dispensed to and stored in on board fuel tanks maintained at a maximum design pressure in the range of about 5000 psi for hydrogen and about 3600 psi for CNG. Higher pressures in the range of about 10,000 psi are likely to be utilized as a result of consumer preference for longer vehicle range after a refill, and as the art progresses. In the instance of a fuel cell powered vehicle, utilization of the hydrogen input into the fuel cell stack occurs at about 30 psi. An increase in the energy efficiency of the overall system of fuel depots and vehicle tanks and refueling systems and their interrelationships is a desired goal.
Hydrogen powered vehicles use high pressure hydrogen storage tanks to store hydrogen on board to power vehicle fuel cell stacks. Herein, reference to high pressure hydrogen powered fuel cell vehicles also correlates the use of the invention with high pressure compressed natural gas powered vehicles. [When the term “hydrogen” is used in the specification, “hydrogen” is a term intended to be interchangeable with CNG in the high pressure environment discussed herein. Namely, both hydrogen and CNG are high pressure gases with which the invention is useful.]
In consumer vehicles, the use of multiple cylindrically shaped small fuel tanks rather than one large tank is preferred for design purposes. Various designs for high pressure hydrogen refueling stations have been proposed to deal with refueling efficiencies. When the on board fuel tanks of a hydrogen powered vehicle are filled with hydrogen, the pressurized on board gas in the tanks may be characterized as having multiple forms of energy: 1) chemical energy associated with the hydrogen fuel itself (consumed in powering the vehicle), and 2) thermodynamic energy, namely, mechanical and thermal energy associated with the physics of high pressure refueling of the on board tank from sources of fuel at the high pressure gas refuel depot.
Hydrogen and CNG fueled vehicles have high pressure on board fuel gas storage tanks. During a high pressure refueling process, the interiors of the on board tanks become heated as a result of fuel gas compression as the tank pressure increases and other refueling parameters affect the refill. After refueling, the interior temperature of the tank and the pressure within the tank both decrease slowly as the fuel gas is consumed during vehicle operation. Conventionally, it is not possible to obtain a full refill tank pressure without pressure compensation during the course of refueling. The charge of fuel pressure input into and stored in the tank must be, at refill (because of the heating compression of the gas), initially in excess of the tank design pressure. Without pressure compensation (an initial overfill), vehicle mileage range is reduced because a full fill is not obtained. When higher optimum tank design pressures are encountered, this condition is exacerbated. In one response to the overfill dilemma, a slower flow rate may be used during refill, which will result in a lower internal tank temperature, and higher pressure, and increased capacity over time. An undesirable consequence of a slower flow rate during refueling to avoid heat build up is self evident—a longer refueling time. Another solution proposes to cool the station fuel gas before refueling; cooling, however, requires substantial energy, thereby reducing the overall efficiency of a hydrogen/CNG economy. Pre-cooling or other pre treatment of the high pressure refuel gas is generally unnecessary when fill pressures are at 5000 psi or lower, however, as pressures approach or exceed 10,000 psi, cooling (and its opposite, internal tank heating by compression) becomes an important factor in the refueling process. A pressure overfill as an option likewise requires additional energy expense where additional gas compression is involved and further increases the heat generated in the tank as a result of high pressure compression during the refill process. In any case, secondary treatment of the refill gas is generally unnecessary when tank fill pressures are at 5000 psi or lower. As tank pressures exceed 3600 psi (for CNG) and 5000 psi and approach or exceed 10,000 psi (for hydrogen), secondary treatment such as cooling becomes an important factor in the refueling process to achieve a full tank capacity fill. When a full fill is achieved, 1) overall vehicle range per each tank refill thereby increases, 2) energy required for a refill (such as for precooling or a pressure overfill) is reduced, 3) time is saved, and 4) overall customer satisfaction increases.